Astrocytes are the most prevalent cell in brain and yet we know virtually nothing about the role of these cells in brain function or disorders. Results from in vitro studies strongly support the concept that the activity of astrocytes is coupled to neuronal activity through neurotransmitter signalling systems. However, no one has shown that astrocytes respond to neurotransmitters released during normal neuronal activity in vivo. The goal of this project is to determine the level of neuronal activity required to activate astrocytes in vivo. It is our premise that a large percentage of neurotransmitter release in vivo may be related to direct signalling between neurons and astrocytes. All available evidence from in vitro studies is consistent with the hypothesis that astrocytes form an extraneuronal signalling system in brain and that this system could profoundly influence the synaptic efficacy of neurons. The long-term goal of our studies is to determine if astrocytes influence neuronal communication in vivo. The vast majority of research on the pharmacology of astrocytes has focused on the responses of astroglia in culture. It is now essential to move beyond cell culture systems and into intact brain tissue to study neuronal-astrocytic interactions in the enormously complex geometry of brain. It is only in the intact brain that the proximity of neurotransmitter release sites, receptors, and inactivation systems remain physiological. It is only here that we can determine if astrocytes respond to neuronal activity and if so, what the function of that response might be. We have chosen to study neuronal-astrocytic communication in the stratum radiatum of CA1 hippocampus. Innervation of CA1 pyramidal neuron apical dendrites in this region occurs primarily through excitatory input from glutamatergic CA3 pyramidal neurons and local inhibitory input from GABAergic interneurons. Confocal microscopy will be used to examine neuronal and astrocytic calcium responses to basal- and stimulus evoked- neurotransmitter release in CA1 s. radiatum. Pharmacological agents will be used to determine whether astrocytic responses reflect neurotransmitter release from neuronal terminals or axons. Developmental studies will be carried out to determine if astrocytic responsiveness changes during brain maturation. The results of these experiments will determine the degree of communication between astrocytes and neurons, the two predominant cell types in brain.